1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member also called a "photoreceptor" and to a method for making such a member or photoreceptor. More specifically, the present invention relates to an electrophotographic photosensitive member having a photoconductive layer made mainly of amorphous silicon.
2. Description of the Prior Art
Recently, amorphous silicon has been used in place of selenium or cadmium sulfide as a photoconductive material constituting a photoconductive layer of an electrophotographic photosensitive member. In comparison with an electrophotographic photosensitive member made mainly of selenium or cadmium sulfide, an electrophotographic photosensitive member made mainly of amorphous silicon has various advantages, for example it has a superior heat resisting property and an abrasion resisting property, it is harmless to its environment and it has a high photosensitivity, among other advantages. Since an electrophotographic photosensitive member made mainly of amorphous silicon has an ample sensitivity even with respect to light of a longer wavelength, it can be used not only in an electrophotographic machine but also in an intelligent copier utilizing a laser printer, an LED printer and the like.
However, in an electrophotographic photosensitive member with amorphous silicon there is a phenomenon which results in a degrading of the surface of the photoconductive layer due to repetition of the electric charging whereby the resolution becomes poor. In case of an electrophotographic photosensitive member having a photoconductive layer made of selenium or cadmium sulfide, the degraded portion is gradually abraded by a cleaning action forming part of the repetition of an electrophotographic process because of a low hardness of the surface whereby it is not necessary to positively abrade the degraded portion. However, in case of an electrophotographic photosensitive member having a photoconductive layer made mainly of amorphous silicon, the surface is hardly abraded by the repetitive electrophotographic processes because the surface of amorphous silicon has a high hardness. Accordingly, it is necessary to abrade the silicon surface to remove the degraded portion each time when several thousand of electrophotographic processes have been completed.
Although a cause of degradation of the surface of a photoconductive layer has not yet been fully clarified, it is presumed that the degradation occurs due to a decrease in the dark resistance. Generally, a photoconductive layer of amorphous silicon is doped with an amount of an additive such as oxygen, nitrogen or the like in order to provide the dark resistance necessary for operating as an electrophotographic photosensitive member. One of the reasons why the dark resistance is reduced is said to be the occurrence of nitrogen ions when the surface of a photoconductive layer is subjected to a corona discharge caused by a high voltage, whereby these nitrogen ions or the like are chemically coupled to the additive or dopant for increasing the dark resistance. Another reason is said to be that nitrogen ions and oxygen ions attached to the surface due to the corona discharge are liable to produce HNO.sub.3. The reason is that HNO.sub.3 adsorbs moisture and the moisture on the surface of a photoconductive layer decreases the resistance on the surface. One possible approach to be considered to prevent a decrease of the dark resistance is to initially dope the material with a large amount of such an additive. However, in such a case, another problem arises namely that the resistance becomes too large and the sensitivity is decreased.